1873
"Theorie des Vogelflugs" (Theory of Bird Flight)

Lilienthal's first known lecture
was a scientific program. Lilienthal deals especially with the
criticism of the balloon and with the necessity of studying bird
flight. He demonstrates wing flapping models. Lilienthal refers to the
"Aeronautical Society of Great Britain", which the Lilienthal brothers
had joined, and to the lack of such a society in Germany: "The
art of flight cannot be invented in the same way that gunpowder was.
For this reason it is a pity that the English and not the more
theoretical Germans had the idea to found an aeronautical society...."

Lilienthal gives a report on models
with engine power. Apart from other things he reports on a mechanical
pigeon that has a coil spring engine and natural wings weighing 50 g
and a steam engine weighing 1500 g producing 1/4 HP that he built
himself.

(The manuscript of the lecture is preserved in the Deutsches Musem (DM) in Munich)

In a series of three lectures in
1888-89 Lilienthal reports the results of his research in bird flight
and measurements of the force of air.
The minutes recorded: "The interest of the Society in
Mr. Lilienthal's interesting experiments was demonstrated by a standing
ovation upon a motion by the chairman."

The lecture reports the essential
part of the book, Lilienthal published in 1889:

The second edition was published in 1910 with an
additional chapter by Gustav Lilienthal.
The third edition was published in 1938 as a facsimile of the first
edition with Lilienthal's corrections in handwriting and with a
foreword by the Professor of Aerodynamics Ludwig Prandtl.
Russian edition "poljot ptiz", St. Petersburg, 1905
English edition (translated from the 2nd edition) "Bird
flight as a Basis of Aviation", New York, 1911

With this book Lilienthal finished
his flight physical preliminary research studies of flight. Now he
believed that he had the necessary knowledge to start building a man
carrying apparatus. In addition to the development of the physics of
flight the book contains a summary of all experiments, that Lilienthal
carried out during the last twenty years, a chapter with rules for the
construction of aeroplanes his further programm, a water-colour, 80
woodcuts from his own hands and two sections of poetry. One of them is
the often quoted poem
about the stork and selections often appear on memorials to
Lilienthal.

Lilienthal presents his knowledge
in articles and lectures in a variety of popular and scientific forms.
The most important lectures take place at the "Deutscher Verein zur
Förderung der Luftschiffahrt" (German Society for Advancement of
Airship Travel) VzFdL. The voice of the society is
the "Zeitschrift für Luftschiffahrt" ZfL (Journal of
Aviation).

In addition, Lilienthal publishes
frequently in the popular weekly magazine "Prometheus".

1890
"Der Flug der Vögel und des Menschen durch die Sonnenwärme"
(The Flight of Birds and Humans By Using the Warmth of the Sun)

Prometheus Nr.55, page 35

Lilienthal
sees essential factors for energy conservation in bird flight not only
in the curved wings but in an ascending wind component. Even though the
title indicates a description of thermal updrafts, Lilienthal sees a
lifting component as a general phenomenon of the wind.

Lilienthal proceeds to experiment
with man-carrying gliders according to his plans described earlier in
his book. Obviously he was dissatisfied with his success: "Free
flight establishes a scope of work for human diligence, that
distinguishes like no other the way that success stands in an
unfavourable ratio to the pains taken over." Lilienthal
starts his article, "But still one thing is certain: we will
not get anywhere just by talking and there is little research to do on
the simple whirling arm device. We have to go on and we must try to be
alone in the air on a real flight…. The transition to flight practise
is therefore a necessary connection for building up our whole knowledge
of aviation."

Lilienthal compares flying to
swimming and cycling in order to make his point: "Even
swimming has a distinctive theory…. Someone who has no knowledge of
swimming and learned the theory well and has done land drills, will
drown in all probability if he is in the situation to use his
theoretical knowledge practically for the first time in order to save
his life."

Shortly afterwards Lilienthal announces the success that
the Frenchman Ferdinand Ferber would later call the beginning of human
flight.

Lilienthal tries to clarify the
laws of physics with strong words: "The unpleasant pamphlets
of aviation hot-headers have expanded the specialized literature with
an enormous number of unproductive thoughts. Ignorance, bigotry and
arrogance have given a disadvantageous character to the whole flight
technical literature…. The layman went on with his emotional mechanics
in abundance and tried to explain the flight process in his way. Of
course all this had to be printed."

In the following years Lilienthal made regular reports
with photographs to the society about his attempts to fly.

A summary is published in "American Engineer and
Railroad Journal" Vol. 7 (1893) at page 342

Lilienthal reports on dangerous
flight situations, the limits of bird flight imitation and the use of
natural wind as he has experienced these in his practical experiments: "What
is the most ingenious man-made flying machine compared to those wings
powered by nature's power? … The imitative wing of man is and always
will be ... a poor instrument and still I can confirm by use of
man-made flying devices that practise and experience will do their part
in order to remove, step by step, certain imperfections and to improve
considerably safety dealing with the wind."
Also, he mentions his most important safety method that he has kept
through his entire flying practise, the possibility of letting oneself
fall out of the apparatus while flying above and against the direction
of the wind onto a smooth slop. "The area surrounding Berlin
is unfortunately poor in good places for practising Sailing Flight. The
ideal is a sandy, round, sloping hill with a height of at least 20
meters, which allows a jump in all directions." In 1894
Lilienthal creates such an artificial hill out of a slagheap near his
flat.

Lilienthal
gives a detailed report on light wing-flapping-engines, which were
created by the Australian Lawrence Hargrave, because adding a
wing-flapping-mechanism was one of Lilienthal's future goals.

ZfL Nr.11, page 259
The article was published in "Aeronautics" Nr.1/7, page 92 as well as
in full by Octave Chanute: "Progress in Flying Machines",
New York, 1894, Appendix
(scanned in G. Bradshaws "to fly is everything")
and in abbreviated form in "L'Aeronaute" 27/1(1894), page 10 as "Essais
de planement dans l'air"

In
November 1893 Lilienthal reports with satisfaction on enormous
progress. Two enclosed pictures show a man flying high above the
landscape of the Rhinow hills. In two other pictures his Berlin
flight-station, a new tower for jumping off, can be seen. After a short
time this building failed to meet his requirements and was replaced
with the "flight hill" in Lichterfelde one year later. In the face of
success he mentions objections by the prominent critics Helmholtz and
Reuleaux: "In those days one had by government decree through
a scholarly commission established that man just can't fly...."

Lilienthal describes:
His new collapsible wing construction;
The principle of the tail of the flying machine and flight control, and
The experience of flight in updraft that carried him higher than he
started.
"I gave up the large wing span of my early gliders step by
step."

He again gives reasons for his
flight position that is essential for his safety on the still unsafe
flights: "It seems to me to be inadvisable to extend the
body in a stretched position …. Later one can possibly turn to that." Extension of flight through a mechanized flapping of the wings is,
apart from flying practise in stronger wind, his next aim: "I
have already finished a steam engine that should move the wings during
my next attempts. It has an overall weight of 20 kg producing 2 hp, and
is being adjusted for an operating time of half an hour."
He reduced the wing camber depth based on his practical experience from
1:18 to 1:20. As the only alternative to the flapping wing he sees
"lifting and at the same time forward moving airscrews". It is
noteworthy is that Lilienthal could not improve flight distance and
control substantially during the next three years before his fatal
crash.

"Zur Flugfrage"(The Problem of Flying)

"Prometheus" Nr. 204, page 753 and Nr. 205, page 769

A
summary of the two above articles appears in "Prometheus". The second
part was published in abbreviated form in the 1893 "Smithsonian
Report," page 189

The first part reports on the
different developmental directions in flight technique from insect
flight to the balloon. The second part continues the report of his work
(The Problem of Flying).
The second part was published as "Practical Experiment in Soaring"
in
the 1893"Smithsonian Report," page 195

"Der Schwebeflug des Menschen" (Man's Sustained
Flight)
is the title of a manuscript that was sold in New York in 1993. The
content is only partly known. One part deals with the project of a
reduced size, man-made flying-hill built in 1894 near Berlin. His
experiences of Rhinow appeared in the article.

Shorter report "Über Schraubenflieger"
(About Propeller Aircraft)

ZfL Nr.12, page 228

Lilienthal analyzes the opinions of
the flight technicians Popper, Lössl and Jarolimek and why he thinks
that propellers, in contrast to flapping wings, are not compatible with
soaring.

"Über den gegenwärtigen Stand der Flugfrage"
(On the current state of the question of Human flight)

On the 22nd of January Lilienthal has given a
lecture with demonstrations of his gliders at the society (VzFdL)
. The name of this lecture is unknown. The page
of a manuscript preserved in the DM "Praktische
Flugversuche - Eine Anleitung zur Entwicklung des freien Fluges"
(Practical Attempts
to Fly - An Instruction for Developing Free Flight) possibly belongs to
this report.

The lecture in front of the "Polytechnische
Gesellschaft" (Polytechnic Society) on the 15th of November was printed
in the magazine "Polytechnisches Zentrallblatt" Nr.56, page 59. The
manuscript is also preserved at the DM.

In a popular lecture, Lilienthal
talks about the ancient dream of human flight, contemporary flight toys
and the studies of bird flight. He shows his flying devices and
compares the problem of flight stability to learning to ride a
unicycle. He gives a precise account of the flights he did: each for
more than 800 feet;
losing 100 feet of heightin in Rhinow, approximately 1,500 during one
year on his "flight-hill" in Lichterfelde.
"In conclusion I want to ask you not to take my achievements
for more than they are. In the photographic pictures, you can see me
flying high above in the sky. One can get the impression that the
problem is already solved. That is not at all the case. I have to admit
that it will still take quite a lot of work to turn this simple gliding
into a long distance human flight. The achievements so far are for
human flight, nothing more than what the first insecure steps of a
child mean to walking of men."
<

Lilienthal gave a very similar lecture to the Society
of Architects Berlin, November 1894:

With concise words he describes the
three problems of flying: take-off, stability, landing (because of the
necessary fast decrease in speed). The problem is the following: "that
you can only learn to fly by practising, but you can only practise
flying without breaking your neck if you understand flying!
That's why the problem of flying is still not solved." From
this fact, he develops his own flight program: "Flying means:
to take off with a flying machine. That we cannot do! Flying means
further: to move through the air from one mountain top to another
similar in height. That we cannot do either! But flying means as well
to let oneself down through the air from the top of a hill to a valley.
And that we can do. ... It is advisable to choose the simplest devices
possible and renounce every motion mechanism. This will lead to ridge
soaring for movement through the air which we must start in our
practical attempts."

Preserved in the DM is a manuscript "Der
Segelflug" (Soaring Flight)
that adopts examples from the aforementioned lectures. The manuscript
deals with the theoretical basis and existing theories of soaring
flight. The manuscript seems to be intended for publishing but appears
incomplete.

In a short announcement Lilienthal reports on
"Maxims Flugmaschine" ("Maxim's Flying Machine")

Lecture held on Feb., 15. at the Berlin Technician Association. Lilienthal reported again on Maxim's and his own machines and compares the suitability of different types of engines.

"Das Flugproblem" (The Problem of Flying)

in "Naturwissenschaftliche-Technisch-Soziale
Korrespondenz"
("Scientific-Technological-Social Correspondence")
The manuscript is preserved.(DM)

In the following articles technical and cultural visions
play a large role. The Lilienthal brothers' great commitment is shown
clearly in the reform movement in Germany around the turn of the
century.
"Yes, If there were already money to be earned in flying,
some would lose their indifference to the mystery of flight. But the
greatest driving force behind technical progress, speculation, is not
yet able to get things moving."
Lilienthal compares human flight with the rapidly developing
electricity technology: "Like vultures swooping down upon the
carcass, … the after-inventors and exploitative industrialists rush to
the great ingenious thoughts."

Another obviously incomplete lecture manuscript is known as:"Über den Fortschritt in der Flugtechnik"
(About the Progress in Flight Technique).

Through mention of the drafts of the airship men Andreè
and Wölfert that were presented at the trade exhibition in Berlin it is
known that the manuscript was written in 1895.
"What
cultural progress could be achieved if we could use the free atmosphere
for air travel, where no mountains, no woods, no water, no marsh
obstruct our movements. You probably have imagined before that borders
would lose their relevance completely, because one cannot barricade
them in the sky. One can hardly imagine that customs duties and wars
could still be possible. The immense progress of world traffic between
peoples would make it necessary that the languages mix to a universal
one. But it should not be my task to talk about the enormous upheaval
in all spheres and to give you an outlook on the future in a way of
enthusiasm. We rather want to see today's standard of flight technique
in a sober light." An analysis of current interest in
balloon technology and of bird flight research follows. The manuscript
ends before it comes to the topic of the title.
On June, 21. Lilienthal gave a lecture in the exhibition Allgemeinen Ausstellung für
Sport Spiel und Turnen (General exhibition for sports,
games and gymnastics). its titel:"Die Fliegekunst als ein Zweig des Turnens"
(Flying - a branch of gymnastics).Lilienthal's
lecture is published in the exhibition catalog and reffered inThe Nursing Record
& Hospital World. London, Vol . XV, No. 383 u. 386, p. 53, 54,
112, 113.

"Der Kunstflug" (Aerobatics)

In the first part, Lilienthal
defines what he called aerobatics: "Artificial
flight may be defined as that form of aviation in which a man flies at
will in any direction, by means of an apparatus, attached to his body,
the use of which requires personal skill. Artificial flight by a single
individual is the proper beginning for all species of artificial
flight. ... The maintenance of equilibrium in forward flight is a
matter of practice, and can only be learned by repeated personal
experiment."
The following report deals with wing profiles and gives instructions
for personal aerobatics in which Lilienthal presents his exercises as a
universally applicable training program.

"Unsere Lehrmeister im Schwebeflug" (Our
Teachers in Soaring Flight)

"Prometheus" Nr.316, page 55
published in "The Aeronautical Annual", 1897, page 85
another handwritten translation by O. Chanute "Our
Masters in Soaring" is preserved in the
archives of the
University of Chicago.

The
article is an evaluation of a trip to study the stork-village Vehlin
(Brandenburg), where Lilienthal counted 40 houses and 54 stork nests.
Accompanied by the photographer Dr. Fülleborn Lilienthal wants to study
the dynamic gliding of storks. Recently several pictures of this trip
were donated to the museum from Lilienthal's estate.

The treatise is a popular account
of the chapter "Aerobatics". Lilienthal's next development would be the
establishment of soaring as a sport, which actually occurred in the
1920's. "It
depends on finding methods to ensure safe attempts to fly and at the
same time, be interesting entertainment for people keen on sports. …
Indescribable is the attraction that flight guarantees. Healthier fresh
air and exercise as well as a more stimulating sport is unimaginable.
The competitive drive during these exercises must lead to continuous
improvement of the flying machines like we experienced with bicycles."
Lilienthal mentions his own experiences that led to a constant
improvement of his gliders. "My experiments basically go in
two directions. On one hand I am trying to extend my successes in
gliding through the air with fixed wing devices as I learn to use
stronger wind to achieve long-lasting flight. On the other hand I am
trying to be successful in dynamic flight with the help of flapping
wings, which are simply added to my gliders. I am not able to present
final results because the necessary equipment still needs improvement."
Lilienthal now has complete command of wind up to 7 m/s. "Nevertheless
I came to the conviction that ... something must be done to perfect
control and easy use of the flying machines." Lilienthal has
what he considers a surprising success with the transition to biplanes.
He can ascend without running from the launching point with a wind
velocity of 10 m/s (36 km/h). "At the highest point of such an flight
path, the glider occasionally comes to a standstill, so that I can
discuss with the gentlemen who would like to take a picture of me about
the right position for the photograph."
With regard to Langley's work ("The Internal Work of the Wind") he
describes almost prophetically future soaring flight: "My
effort…turns to… following and circling in the strong lifting breeze."
Finally, he makes an appeal to enlarge his "flight-hill" with private
money: "Both the state in Moscow and private people in Boston
are interested in establishing an institution for private flight
testing on a large scale. It would be a pity if something like that
can't be done in our fatherland because of a lack of entrepreneurial
spirit."

ZfL Nr.14/2,3; page 42
An extract was published as "At Rhinow" in "The
Aeronautical Annual" 1897, page 92

Lilienthal sums up the results of
his experiments of wing profiles (airfoils) and some new findings: "Nature
seems to attach great importance to a smooth upper surface, at least
you can see that clearly in the shape of the remige (flight feathers)
that are always totally smooth. … From this one can conclude that the
suction effect above the wings is of more importance than the pressure
effect of the air on the underside of the wing."

in ZfL Nr.14/10, page 237
published as "La découverte des meilleures formes d' ailes"
in "L'Aéronaute" 29/1 (1896), page 5
and as "K woprosu o mechanitschekoi letanii"
translated from French into Russian with an obituary in "Inshenernyi
shurnal" Nr.40 (Okt.1896), page 122
An extract was published as "The Best Shape of Wings" in "The Aeronautical Annual", 1897, page 95. The extract is about
planned model experiments that are called "Lilienthal's Unfinished
Work".

"That the feather-made
structure of a wing gives special advantages to it that improves the
lifting force, is already assumed but not yet proved. … That is why it
is doubtful that we are doing something wrong when we stick to the
easier, more practical construction of the bat wing when building
flying machines."
Lilienthal refers to new bird research and to the usefulness of a
thickening of the profile at the leading edge. The work on the wing
profile is more important than the engine. For his further work, he
sees two directions: the better control of the wing and the driving
device. "The time-consuming improvements … of the machine
equipment didn't lead me to specific results. On the other hand I was
on better terms with the wind last summer..."
As an addition to the article "Flying - Sport and Experience,"
Lilienthal mentions his experimental apparatus from 1895 (wing warping,
air resistance winglets, pre wing flap): "I am rejecting
methods that should, through arbitrary changes in the form of the
wings, increase the stability of flight, because other principles
surprisingly have shown more promising results.": the
biplane.
For optimising wing profiles (airfoils), he describes a program with
models that is superior in accuracy when compared with well-known
measuring methods: "I make these profiles out of stiff
drawing paper, approximately 10 cm wide and 50 cm long."
These should be launched from a high point in calm weather. By
averaging the results of several measurements of the flight times the
best profile should be determined.

1896
"Praktische Flugversuche" (Practical Attempts to Fly)

Lecture
at the "Trade Exhibition" in Berlin on the 10th of June where
Lilienthal displayed the products of his steam engine factory. It is
the last known lecture before his crash.

"Der Schwebeflug des Menschen" (Soaring Flight of People)

is the title of the last known newspaper article by Otto Lilienthal. He advertises for a flight sport at an artificial fly mountain of 25 meters.

screenplay for the "Ostendtheater. Lilienthal was a
co-owner of this "peoples theatre".

In the years between 1890
and 1895, Lilienthal wrote several book reviews,
articles, and reports for the ZfL and for the
popular "Prometheus". Articles signed with "L." and "O.L." are ascribed
to Lilienthal as well.